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(meteorobs) My last word on the Leonids outlook 2001
Here is the latest write-up of what four different models
predict for the Leonids next week - I've tried to be fair
towards all parties ... This article can be found at
http://www.geocities.com/skyweek/mirror/230.html with
many links - to get 'into the mood' of meteor storms, I
especially recommend the eyewitness reports linked there!
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Expect 7000+ Leonids per hour, (some) experts agree
With the 2001 return of the Leonids approaching fast, the
discrepancies between four detailled models that had
become evident this summer have not been resolved at all.
All of the models have been published in great detail
in the literature now: Two of them predict a major meteor
storm of 7000 to 9000 meteors per hour over Eastern Asia
and Australia, together with a lesser outburst over America,
one predicts nice storms over both America and the Far East -
and one sees just one very extended maximum of activity that
barely reaches storm level (if defined as 1000 meteors/hour)
over the Pacific Ocean.
All four models are firmly based on the existence of dust
trails, created by the parent comet during every perihelion
passage, which have permitted successful predictions of
the Leonids storm of 1999 and of the general activity profile
in 2000. But efforts to further improve the modelling have
been moving into different directions, with the resulting
predictions for 2001 diverging to a surprising degree. And it's
not just the strengths of the maxima that differ between the
models, also the times of the maxima are no longer identical
for different approaches. On November 19 we will know who
did the best job:
An outburst or small storm over America plus a
big storm over Asia & Australia is the prediction
by the celebrated theoreticians who had
re-discovered the dust trail idea in 1999. Robert
McNaught & David Asher are now including aging
effects of the dust trails, reducing the strengths of
the maxima somewhat. Here are their predictions
(the time of the maximum on Nov. 18, how many
revolutions the respective dust trail is old, the
expected maximum Zenithal Hourly Rate, and for
how many minutes before or after the peak the rate
will be still at least half the maximum value):
9:55 UTC | 7 rev. | max. 800 | ±45 m
17:24 UTC | 9 rev. | max. 2000 | ±65 m
18:13 UTC | 4 rev. | max. 8000 | ±35 m
The two maxima around 18 UTC will overlap and
cause a maximum overall rate of some 9000 around
18:10 UTC, twice as high as during the 1999 storm.
But the 10 UT maximum won't reach storm level
any more in this model.
Esko Lyytinen et al. are worried primarily about the
role of non-gravitational forces on the orbits of the
dust trails which can also cause significant shifts of
the times of the maxima. They predict:
10:28 UTC | 7 rev. | max. 2000 | ±29 m
18:03 UTC | 9 rev. | max. 2600 | ±31 m
18:20 UTC | 4 rev. | max. 5000 | ±21 m
Again the last two maxima will overlap, generating a
combined peak ZHR of about 7200, just before 18:20
UTC. And the Americas could still hope for a small
storm, albeit at only half the strength of the 1999
event.
A bigger storm over the Americas than over Asia
is the prediction by Peter Jenniskens who has
drastically revised his forecasts for 2001 and 2002
repeatedly since mid-2000. He sees evidence
(disputed by others) that some of the dust trails have
shifted towards the Sun, which would not affect the
times but the relative strengths of the 2001 maxima:
10:09 UTC | 7 rev. | max. 4200 | ±20 m
17:08 UTC | 9 rev. | max. 1800 | ±37 m
17:21 UTC | 11 rev. | max. 510 | ±38 m
17:55 UTC | 4 rev. | max. 2700 | ±24 m
The combined power of the three trails to be
encountered from 17 to 18 UTC (the 11 rev. trail has
less effect in the other two models) would cause a
prolonged peak of perhaps 3000, while the Americas
would face a short peak of over 4000.
A very extended shallow peak that barely reaches
storm level at no more that 1500 meteors/hour
between 12 and 13 UTC is the forecast by Peter
Brown & Bill Cooke who have simulated the orbits
of millions of dust particles ejected from the comet
under various assumptions. The very different
outcome of these calculations (neither the 10 UTC
nor the 18 UTC peak are pronounced here) seems to
come from the greater degrees of freedom that the
particles have to drift away from the dust trails in
this 3D world.
There are only three areas in which all four models agree:
a) There will be at least one meteor storm this year, some
time between 9 UTC and 19 UTC on Nov. 18, b) there will
not be any storms over Europe, Africa and Western Asia,
and c) the fluence of meteoroids, i.e. the number of dust
particles hitting the atmosphere per area per time, will be
5 to 10 times larger this time than in 1999. Thus there is a
somewhat higher risk for orbiting satellites, but it's not as
dramatic as stated in some media reports. (McNaught &
Asher, WGN 29 [Oct. 2001] 156-164, Lyytinen et al., ibid.
29 [Aug. 2001] 110-8, Jenniskens, ibid. 29 [Oct. 2001]
165-175, and Brown & Cooke, MNRAS 326 [Sep. 11,
2001] L19-22)
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Daniel Fischer, Koenigswinter, Germany (now off to
Korea, to find out who's got it right this time ...)
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